The present invention has its genesis from consideration of the problems faced by aircraft designers in monitoring the integrity of: sandwiched structures typically encountered around splices and cut-outs in fuselages; and, substantially hollow components as encountered in composite structures such as flaps, doors, panels and the like; and, attempting to prevent the ingress of moisture into such structures and components. These structures and components are difficult to examine for the detection of cracking, corrosion and disbonding. Further they are prone to the ingress of moisture arising for various reasons including: capillary action and the substantially hollow nature of the structures, particularly those made from composite materials; exposure to temperature extremes; exposure to large ambient pressure variations; exposure to environments of high humidity and precipitation.
Apart from corrosion in metallic structure, the ingress of moisture can lead to serious structural flaws such as disbanding due to progressive damage caused by the cyclic intrusion of the moisture followed by expansion as it freezes.
Of course the above problems are not the exclusive domain of aircraft designers. Structural integrity monitoring has very wide application and can be used, for example, to monitor the adhesive bonds such as between anechoic tiles on a submarine or heat resistant tiles on a spacecraft.